As is well known in the art, production of hydrocarbons from subsurface formations typically entails using a drill-bit to drill a borehole that reaches the desired subsurface formation. In most cases, the bit is at the remote end of a length of tubing and drills a borehole that is somewhat larger than the tubing diameter, forming an annulus between the borehole and the outside of the tubing. Drilling fluid, also referred to as “mud,” is pumped down the tubing, flows out through the bit, and returns to the surface via the annulus, carrying with it the cuttings from the borehole bottom. The mud density or “mud weight” may vary for a number of reasons, including but not limited to changes in the quantity and density of cuttings; changes in the pressure applied at the surface, changes in temperature, etc.
Variations in mud density may also occur when gas or liquid enter the borehole from the formation. Because the formation fluid is unlikely to have the same density as the mud in the hole, such influx, known as a “kick,” is likely to cause a change in the pressure in the annulus. By way of example, if formation fluids having a significantly lower density than the drilling mud flow into the annulus and displace the mud therein, the pressure at the bottom of the hole will drop. If not controlled, this may in turn cause an unexpected flow of formation fluids to the surface, sometimes referred to as a “blowout.”
Underbalanced drilling, in which the mud pressure at the bottom of the hole is less than the formation pressure, can cause a kick. At the same time, an overbalance of mud pressure versus formation pressure tends to decrease the drilling rate and increase lost circulation and differential sticking. Thus, balanced drilling often allows only a small margin between effective pressure control and a threatened blowout and influx detection is an important aspect of drilling control.
Some common techniques for detecting unexpected changes in formation pressure are based on measurement of drilling parameters such as drilling rate, torque and drag; drilling mud parameters such as mud gas, cuttings, flow line mud weight, flow line temperature, mud pit level, and mud flow rate; and shale cutting parameters such as bulk density, shale factor, volume and size of cuttings. A drawback of some of these measurements is that they are not available in real-time because of the need to wait while fluid from the hole bottom returns to the surface. Other known methods for identifying possible kicks rely on density measurements of the borehole fluid. A drawback of these methods they are not always sufficiently sensitive to provide warning of an imminent gas kick.
Generally available kick detection systems are designed primarily for detecting kicks during pumps-on activities. Nonetheless, a kick may occur while the mud pumps are turned off, e.g. during the time required to add another length of pipe; also known as making a connection. During a pumps off event, bottom hole pressure in the wellbore will decrease due to loss of the frictional component of total equivalent circulating density (ECD). ECD being made up of three components; static fluid density, cuttings loading density and return annulus frictional pressure (expressed as equivalent density) exerted when pumps are running. The mud flow out of the well will transition (over a period of seconds or minutes) from normal pumps on flow rate to zero. If there is a change in the normal shape of the transient mud flow out response, after pumps stopped, this could indicate formation influx into wellbore.
Regardless of the criteria they use, most existing influx or kick detection systems require interaction with an operator to perform successfully. For example, it is not uncommon for a system to require manual adjustment of alarm settings in order to keep up with changes in well conditions. In order to decrease response time and to reduce or eliminate the possibility of human error, it would be desirable to provide a system that operates automatically.
Thus, a need remains for a system and method for accurately and automatically predicting imminent kicks and for detecting kicks during pumps-off events.